CVE-2018-20856

What it actually does

Use-after-free in the Linux kernel block layer (__blk_drain_queue in blk-core.c) due to mishandled error case, enabling memory corruption and potential arbitrary code execution with kernel privileges.

Why it matters to the business

Local attackers on affected Linux systems could gain elevated privileges or cause denial of service, risking data integrity, system stability, and compliance on servers, embedded devices, and cloud workloads running vulnerable kernels.

How an attack unfolds

An attacker with local access triggers a specific error path during block queue draining to cause the use-after-free; the freed memory is then reused for kernel-level code execution or privilege escalation.

How to fix it

Upgrade the Linux kernel to version 4.18.7 or newer; apply commit 54648cf1ec2d7f4b6a71767799c45676a138ca24 from the mainline repository.

How to confirm the fix worked

Confirm the running kernel version is 4.18.7 or later and verify the fix commit is present in the kernel source tree.

Typical attack path

1. Gain local user access to a vulnerable Linux system
2. Craft or trigger I/O operations that hit the mishandled error case in blk-core.c
3. Leverage the resulting use-after-free to corrupt kernel memory
4. Escalate to root or execute arbitrary kernel code

How to detect it

Review kernel logs and dmesg output for block layer errors or oops messages around suspected incidents; inspect core dumps or memory forensics for signs of corruption in queue structures.

What your security team should do

• Prioritize kernel updates on all exposed Linux systems
• Restrict local user access and harden block device permissions where feasible
• Monitor kernel crash reports and audit logs for suspicious block I/O activity
• Test patches in staging environments before broad deployment